1. Field of the Invention
This invention relates to the manufacture of semiconductor integrated circuit packages, and more particularly to the manufacture of semiconductor integrated circuit packages incorporating a transparent window, which are commonly used to encase erasable programmable read only memory ("EPROM") devices.
2. Description of Related Art
In recent years, EPROM devices have been encased in semiconductor integrated circuit ("IC") packages which incorporate a window that is transparent to certain wave-lengths of radiation commonly used to erase an EPROM device, such as ultraviolet ("UV") radiation. The inclusion of the window permits data stored on an EPROM device to be erased or rewritten by directing ultraviolet radiation through the window and onto the enclosed EPROM circuit.
Substantial economies have resulted from the use of the windowed IC package, since the enclosed EPROM circuit need not be discarded and replaced but simply erased and reprogrammed when new data is required.
Generally, windowed IC packages are made of ceramic and glass and are comprised of two basic components: (1) a base incorporating an embedded lead frame surrounding a central cavity in which the EPROM circuit will ultimately be affixed; and (2) a ceramic lid for the base incorporating a transparent window. The base and lid are manufactured separately and sealed together after an EPROM circuit has been mechanically affixed to the interior of the cavity of the base and electrically connected to the lead frame.
Commonly, the ceramic lid is manufactured in a three-step process: (1) a substrate, which ordinarily conforms to the size and shape of the base, is created incorporating a central opening corresponding to the position of the base; (2) a window, often made of glass, sapphire or quartz, and which is transparent to ultraviolet radiation, is affixed to the substrate so as to plug and seal the central opening; (3) the interior surface of the substrate, exclusive of the window and its surrounding area, is screened with a sealing glass material, which when attached to the base and heated to high temperatures will seal the lid to the base.
A disadvantage of the prior art has been that during the manufacturing, packing and shipping processes involving the lid, especially the process during which the sealing glass material is screened onto the substrate, foreign particulate matter collects in the vicinity of the window and often adheres to the window. Such foreign particulate matter often remains on the interior surface of the lid and the window despite efforts to clean them before the lid and the base are sealed together. Once the EPROM circuit has been affixed to the base and the base and lid sealed, the foreign particulate matter can have several deleterious effects. First, since line widths on high-density EPROM circuits are now less than one micron, even relatively small foreign particles falling from the interior surface of the lid or the window and landing on the EPROM device may bridge across circuit traces resulting in undesirable short circuits, and thus impair the ability to reprogram the EPROM circuit once the prior data has been erased. Second, foreign particulate matter adhering to the window may totally shield or partially obscure the EPROM circuit from ultraviolet radiation directed through the window, thereby impairing the ability to fully erase old data.
Accordingly, as the semiconductor industry moves forward to manufacturing more highly integrated EPROM devices, there is an ever-increasing need to minimize or eliminate the noted contamination problems and to provide a ceramic lid having a high-grade UV transparent window suitable for use in a high-density EPROM package.